Ischemic heart disease is the major cause of death in industrialized countries. Ischemic heart disease, which often results in myocardial infarction, is a consequence of coronary atherosclerosis. Atherosclerosis is a complex chronic inflammatory disease and involves focal accumulation of lipids and inflammatory cells, smooth muscle cell proliferation and migration, and the synthesis of extracellular matrix. Nature 1993;362:801–809. These complex cellular processes result in the formation of atheromatous plaque, which consists of a lipid-rich core covered with a collagen-rich fibrous cap, varying widely in thickness. Further, plaque disruption is associated with varying degrees of internal hemorrhage and luminal thrombosis because the lipid core and exposed collagen are thrombogenic. J Am Coll Cardiol. 1994;23:1562–1569 Acute coronary syndrome usually occurs as a consequence of such disruption or ulceration of a so called “vulnerable plaque”. Arterioscler Thromb Vasc Biol. Volume 22, No. 6, June 2002, p. 1002.
In addition to coronary bypass surgery, a current treatment strategy to alleviate vascular occlusion includes percutaneous transluminal coronary angioplasty, expanding the internal lumen of the coronary artery with a balloon. Roughly 800,000 angioplasty procedures are performed in the U.S. each year (Arteriosclerosis, Thrombosis, and Vascular Biology Volume 22, No. 6, Jun. 2002, p. 884). However, 30% to 50% of angioplasty patients soon develop significant restenosis, a narrowing of the artery through migration and growth of smooth muscle cells.
In response to the significant restenosis rate following angioplasty, percutaneously placed endoprostheses have been extensively developed to maintain fluid flow through a diseased coronary artery. Such endoprostheses, or stents, which have been traditionally fabricated using metal alloys, include self-expanding or balloon-expanded devices that are “tracked” through the vasculature and deployed proximate one or more lesions. Stents considerably enhance the long-term benefits of angioplasty, but 10% to 50% of patients receiving stents still develop restenosis. (J Am Coll Cardiol. 2002; 39:183–193. Consequently, a significant portion of the relevant patient population undergoes continued monitoring and, in many cases, additional treatment.
Continued improvements in stent technology aim at producing easily tracked, easily visualized and readily deployed stents, which exhibit the requisite radial strength without sacrificing a small delivery profile and sufficient flexibility to traverse the diseased human vasculature. Further, numerous therapies directed to the cellular mechanisms of accumulation of inflammatory cells, smooth muscle cell proliferation and migration show tremendous promise for the successful long-term treatment of ischemic heart disease. Consequently, advances in coupling delivery of such therapies to the mechanical support of vascular endoprostheses, delivered proximate the site of disease, offer great hope to the numerous individuals suffering heart disease.
While advances in the understanding of ischemic heart disease as a complex chronic inflammatory process take place, traditional diagnostic techniques such as coronary angiography yield to next generation imaging modalities. In fact, coronary angiography may not be at all useful in identifying inflamed atherosclerotic plaques that are prone to producing clinical events. Imaging based upon temperature differences, for example, are undergoing examination for use in detecting coronary disease. Magnetic resonance imaging (MRI) is currently emerging as the state of the art diagnostic arterial imaging, enhancing the detection, diagnosis and monitoring of the formation of vulnerable plaques. Transluminal intervention guided by MRI is expected to follow. However, metals produce distortion and artifacts in MR images, rendering use of the traditionally metallic stents in coronary, biliary, esophageal ureteral, and other body lumens incompatible with the use of MRI.
Consequently, an emerging clinical need for interventional devices that are compatible with and complementary to new imaging modalities is evident. Further, devices that exhibit improved trackability to previously undetectable disease within remote regions of the body, especially the coronary vasculature are needed. And finally, devices that both exhibit improved mechanical support and are readily compatible with adjunct therapies in order to lower or eliminate the incidence of restenosis are needed.